A variety of industrial and research fields require a novel method for generating fine particles of functional multi-element-ceramics which have the same functionality as the bulk material they are derived from. These materials include phosphorous ceramics, light-absorbing ceramics including sunlight absorbing materials in solar cells, and ceramic ionic conductors which can be used in a fuel cell.
Generally, to generate fine particles there are two approaches. First, is a synthetic approach like a chemical wet process where the generation starts with starting materials whose chemical identity is different from that of the final synthesized product. Second, is a process wherein the starting material is ground down to fine particle sizes such as in a mechanical milling process. This process can damage the chemical properties and crystalline structure of the starting material.
Luminescence is defined as a phenomenon in which a material absorbs external energy and then to emits light, preferably visible light. In a broad sense, a material exhibiting this phenomenon is called a “phosphor”. Currently, phosphors are being used in many devices. This list includes but is not limited to: field emission devices, plasma display panels, cathode ray tubes (CRTs), light emission diodes (LEDs), vacuum florescent displays (VFDs), electro luminescence displays (ELDs), RGB screens, radiographic imaging, nuclear spectroscopy, crystal scintillators, and biotags.
In many industries, such as the LED industry, there is a need for small-sized phosphor particles, also called fine phosphors. One of the main reasons is to make it easy to handle them in printing, painting, coating, molding or spraying applications and to lower production costs of phosphor material since some phosphors contain expensive rare-earth materials. Small phosphor particles are advantageous for making a packed thin layer of phosphor coating with a minimum volume. Also, it is expected that higher emission efficiency, which is the ratio of emitted optical photons to absorbed energy, will be achieved by particle downsizing. It is known that reduction of the phosphor size to the nanometer scale of 1 to 100 nanometers, i.e., nanophosphor, alters its properties. Nanophosphors, in general, have very good emission efficiency.
There are several methods of making fine phosphors including nanophosphors, such as spray pyrolysis, sol-gel, hydrothermal synthesis, chemical vapor synthesis, and solvothermal synthesis in addition to conventional milling processes where the particle size is in micron range. See for example: D. Dosev, Bing Guo and I. M. Kennedy, “Photoluminescence of Eu3+: Y2O3 as an indication of crystal structure and particle size in nanoparticles synthesized by flame spray pyrolysis”, Aerosol Science Vol. 37, 402, 2006; D. Jia, “Nanophosphors for White Light LEDs”, Chem. Eng. Comm. Vol. 194, 1666, 2007; H. Zhu, E. Zhu, H. Yang, L. Wang, D. Jin and K. Yao, “High-Brightness LaPO4: Ce3+, Tb3+ Nanophosphor: Reductive Hydrothermal Synthesis and Photoluminescent Properties”, J. Am. Ceram. Soc. Vol. 91, 1682, 2008; A. Konrad, T. Fries, A. Gahn, F. Kummer, U. Herr, R. Tidecks and K. Samwer, “Chemical vapor synthesis and luminescence properties of nanocrystalline cubic Y2O3; Eu”, J. Appl. Phys. Vol. 86, 3129, 1999; X. Li, H. Liu, J. Wang, H. Cui, S. Yang and I. R. Boughton, “Solvothermal synthesis and luminescent properties of YAG: Tb nano-sized phosphor”, Journal of Physics and Chemistry of Solids. Vol. 66, 201, 2005; and JP3690968. However, the above methods are limited by the material that can be used either due to solubility and/or hardness limitations. Also, the non-mechanical methods do not generate fine phosphors directly and require additional treatment steps to generate the fine phosphors.